Liquid-crystalline medium

ABSTRACT

The invention relates to a liquid-crystalline medium comprising one or more compounds of the formula I 
     
       
         
         
             
             
         
       
     
     in which
 
R 0 , ring A and L 1-4  have the meanings indicated in claim  1 , and to the use thereof in electro-optical liquid-crystal displays.

The present invention relates to a liquid-crystalline medium (LCmedium), to the use thereof for electro-optical purposes, and to LCdisplays containing this medium.

Liquid crystals are used principally as dielectrics in display devices,since the optical properties of such substances can be modified by anapplied voltage. Electro-optical devices based on liquid crystals areextremely well known to the person skilled in the art and can be basedon various effects. Examples of such devices are cells having dynamicscattering, DAP (deformation of aligned phases) cells, guest/host cells,TN cells having a twisted nematic structure, STN (supertwisted nematic)cells, SBE (super-birefringence effect) cells and OMI (optical modeinterference) cells. The commonest display devices are based on theSchadt-Helfrich effect and have a twisted nematic structure. Inaddition, there are also cells which work with an electric fieldparallel to the substrate and liquid-crystal plane, such as, forexample, IPS (in-plane switching) cells. TN, STN, FFS (fringe fieldswitching) and IPS cells, in particular, are currently commerciallyinteresting areas of application for the media according to theinvention.

The liquid-crystal materials must have good chemical and thermalstability and good stability to electric fields and electromagneticradiation. Furthermore, the liquid-crystal materials should have lowviscosity and produce short addressing times, low threshold voltages andhigh contrast in the cells.

They should furthermore have a suitable mesophase, for example a nematicor cholesteric mesophase for the above-mentioned cells, at the usualoperating temperatures, i.e. in the broadest possible range above andbelow room temperature. Since liquid crystals are generally used asmixtures of a plurality of components, it is important that thecomponents are readily miscible with one another. Further properties,such as the electrical conductivity, the dielectric anisotropy and theoptical anisotropy, have to satisfy various requirements depending onthe cell type and area of application. For example, materials for cellshaving a twisted nematic structure should have positive dielectricanisotropy and low electrical conductivity.

For example, for matrix liquid-crystal displays with integratednon-linear elements for switching individual pixels (MLC displays),media having large positive dielectric anisotropy, broad nematic phases,relatively low birefringence, very high specific resistance, good UV andtemperature stability and low vapour pressure are desired.

Matrix liquid-crystal displays of this type are known. Non-linearelements which can be used to individually switch the individual pixelsare, for example, active elements (i.e. transistors). The term “activematrix” is then used, where a distinction can be made between two types:

-   1. MOS (metal oxide semiconductor) or other diodes on silicon wafers    as substrate.-   2. Thin-film transistors (TFTs) on a glass plate as substrate.

The use of single-crystal silicon as substrate material restricts thedisplay size, since even modular assembly of various part-displaysresults in problems at the joints.

In the case of the more promising type 2, which is preferred, theelectro-optical effect used is usually the TN effect. A distinction ismade between two technologies: TFTs comprising compound semiconductors,such as, for example, CdSe, or TFTs based on polycrystalline oramorphous silicon. Intensive work is being carried out worldwide on thelatter technology.

The TFT matrix is applied to the inside of one glass plate of thedisplay, while the other glass plate carries the transparentcounterelectrode on its inside. Compared with the size of the pixelelectrode, the TFT is very small and has virtually no adverse effect onthe image. This technology can also be extended to fully colour-capabledisplays, in which a mosaic of red, green and blue filters is arrangedin such a way that a filter element is opposite each switchable pixel.

The TFT displays usually operate as TN cells with crossed polarisers intransmission and are backlit.

The term MLC displays here encompasses any matrix display withintegrated non-linear elements, i.e., besides the active matrix, alsodisplays with passive elements, such as varistors or diodes(MIM=metal-insulator-metal).

MLC displays of this type are particularly suitable for TV applications(for example pocket televisions) or for high-information displays forcomputer applications (laptops) and in automobile or aircraftconstruction. Besides problems regarding the angle dependence of thecontrast and the response times, difficulties also arise in MLC displaysdue to insufficiently high specific resistance of the liquid-crystalmixtures [TOGASHI, S., SEKIGUCHI, K., TANABE, H., YAMAMOTO, E.,SORIMACHI, K., TAJIMA, E., WATANABE, H., SHIMIZU, H., Proc. Eurodisplay84, September 1984: A 210-288 Matrix LCD Controlled by Double StageDiode Rings, pp. 141 ff., Paris; STROMER, M., Proc. Eurodisplay 84,September 1984: Design of Thin Film Transistors for Matrix Addressing ofTelevision Liquid Crystal Displays, pp. 145 ff., Paris]. With decreasingresistance, the contrast of an MLC display deteriorates, and the problemof after-image elimination may occur. Since the specific resistance ofthe liquid-crystal mixture generally drops over the life of an MLCdisplay owing to interaction with the interior surfaces of the display,a high (initial) resistance is very important in order to obtainacceptable service lives. In particular in the case of low-voltmixtures, it was hitherto impossible to achieve very high specificresistance values. It is furthermore important that the specificresistance exhibits the smallest possible increase with increasingtemperature and after heating and/or UV exposure. The low-temperatureproperties of the mixtures from the prior art are also particularlydisadvantageous. It is demanded that no crystallisation and/or smecticphases occur, even at low temperatures, and the temperature dependenceof the viscosity is as low as possible. The MLC displays from the priorart thus do not satisfy today's requirements. Besides liquid-crystaldisplays which use backlighting, i.e. are operated transmissively and ifdesired transflectively, reflective liquid-crystal displays are alsoparticularly interesting. These reflective liquid-crystal displays usethe ambient light for information display. They thus consumesignificantly less energy than backlit liquid-crystal displays having acorresponding size and resolution. Since the TN effect is characterisedby very good contrast, reflective displays of this type can even be readwell in bright ambient conditions. This is already known of simplereflective TN displays, as used, for example, in watches and pocketcalculators. However, the principle can also be applied to high-quality,higher-resolution active matrix-addressed displays, such as, forexample, TFT displays. Here, as already in the trans-missive TFT-TNdisplays which are generally conventional, the use of liquid crystals oflow birefringence (Δn) is necessary in order to achieve low opticalretardation (d·Δn). This low optical retardation results in usuallyacceptable low viewing-angle dependence of the contrast (cf. DE 30 22818). In reflective displays, the use of liquid crystals of lowbirefringence is even more important than in transmissive displays sincethe effective layer thickness through which the light passes isapproximately twice as large in reflective displays as in transmissivedisplays having the same layer thickness.

For TV and video applications, displays having fast response times arerequired in order to be able to reproduce multimedia content, such as,for example, films and video games, in near-realistic quality. Suchshort response times can be achieved, in particular, if liquid-crystalmedia having low values for the viscosity, in particular the rotationalviscosity γ_(i), and having high optical anisotropy (Δn) are used.

Thus, there continues to be a great demand for MLC displays having veryhigh specific resistance at the same time as a large working-temperaturerange, short response times, even at low temperatures, and a lowthreshold voltage which do not exhibit these disadvantages or only do soto a lesser extent.

In the case of TN (Schadt-Helfrich) cells, media are desired whichfacilitate the following advantages in the cells:

-   -   extended nematic phase range (in particular down to low        temperatures)    -   the ability to switch at extremely low temperatures (outdoor        use, automobiles, avionics)    -   increased resistance to UV radiation (longer lifetime)    -   low threshold voltage.

The media available from the prior art do not enable these advantages tobe achieved while simultaneously retaining the other parameters.

In the case of supertwisted (STN) cells, media are desired whichfacilitate greater multiplexability and/or lower threshold voltagesand/or broader nematic phase ranges (in particular at low temperatures).To this end, a further widening of the available parameter latitude(clearing point, smectic-nematic transition or melting point, viscosity,dielectric parameters, elastic parameters) is urgently desired.

In particular in the case of LC displays for TV and video applications(for example LCD TVs, monitors, PDAs, notebooks, games consoles), asignificant reduction in the response times is desired. A reduction inthe layer thickness d (“cellap”) of the LC medium in the LC celltheoretically results in faster response times, but requires LC mediahaving higher birefringence Δn in order to ensure an adequate opticalretardation (dΔn). However, the LC materials of high birefringence knownfrom the prior art generally also have high rotational viscosity at thesame time, which in turn has an adverse effect on the response times.There is therefore a demand for LC media which simultaneously have fastresponse times, low rotational viscosities and high birefringence.

The invention is based on the object of providing media, in particularfor MLC, TN, STN, FFS or IPS displays of this type, which have thedesired properties indicated above and do not exhibit the disadvantagesmentioned above or only do so to a lesser extent. In particular, the LCmedia should have fast response times and low rotational viscosities atthe same time as high birefringence. In addition, the LC media shouldhave a high clearing point, high dielectric anisotropy and a lowthreshold voltage.

It has now been found that this object can be achieved if LC mediacomprising one or more compounds of the formula I are used. Thecompounds of the formula I result in mixtures having the desiredproperties indicated above.

The invention relates to a liquid-crystalline medium, characterised inthat it comprises one or more compounds of the formula I

in which

-   R⁰ denotes an alkyl or alkoxy radical having 1 to 15 C atoms, where,    in addition, one or more CH₂ groups in these radicals may each be    replaced, independently of one another, by —C≡C—, —CF₂O—, —CH═CH—,

—O—, —CO—O— or —O—CO— in such a way that O atoms are not linked directlyto one another, and in which, in addition, one or more H atoms may bereplaced by halogen,

denotes

-   L¹⁻⁴ each, independently of one another, denote H or F.

Surprisingly, it has been found that LC media comprising compounds ofthe formula I have a very good ratio of rotational viscosity γ_(i) andclearing point, a high value for the optical anisotropy Δ∈ and highbirefringence Δn, as well as fast response times, a low thresholdvoltage, a high clearing point, high positive dielectric anisotropy anda broad nematic phase range. Furthermore, the compounds of the formula Iare very readily soluble in liquid-crystalline media.

The compounds of the formula I have a broad range of applications.Depending on the choice of substituents, they can serve as basematerials of which liquid-crystalline media are predominantly composed;however, liquid-crystalline base materials from other classes ofcompound can also be added to the compounds of the formula I in order,for example, to modify the dielectric and/or optical anisotropy of adielectric of this type and/or to optimise its threshold voltage and/orits viscosity.

Preferred compounds of the formula I are mentioned below:

in which R⁰ has the meanings indicated above. R⁰ preferably denotes astraight-chain alkyl radical.

In the pure state, the compounds of the formula I are colourless andform liquid-crystalline mesophases in a temperature range which isfavourably located for electro-optical use. They are stable chemically,thermally and to light.

The compounds of the formula I are prepared by methods known per se, asdescribed in the literature (for example in the standard works, such asHouben-Weyl, Methoden der organischen Chemie [Methods of OrganicChemistry], Georg-Thieme-Verlag, Stuttgart), to be precise underreaction conditions which are known and suitable for the said reactions.Use can also be made here of variants known per se, which are notmentioned here in greater detail. The compounds of the formula I arepreferably prepared as follows:

Particularly preferred compounds are prepared in accordance with thefollowing scheme:

If R⁰ in the formulae above and below denotes an alkyl radical and/or analkoxy radical, this may be straight-chain or branched. It is preferablystraight-chain, has 2, 3, 4, 5, 6 or 7 C atoms and accordinglypreferably denotes ethyl, propyl, butyl, pentyl, hexyl, heptyl, ethoxy,propoxy, butoxy, pentoxy, hexyloxy or heptyloxy, furthermore methyl,octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl,methoxy, octyloxy, nonyloxy, decyloxy, undecyloxy, dodecyloxy,tridecyloxy or tetradecyloxy.

Oxaalkyl preferably denotes straight-chain 2-oxapropyl (=methoxymethyl),2-(=ethoxymethyl) or 3-oxabutyl (=2-methoxyethyl), 2-, 3- or4-oxapentyl, 2-, 3-, 4- or 5-oxahexyl, 2-, 3-, 4-, 5- or 6-oxaheptyl,2-, 3-, 4-, 5-, 6- or 7-oxaoctyl, 2-, 3-, 4-, 5-, 6-, 7- or 8-oxanonyl,2-, 3-, 4-, 5-, 6-, 7-, 8- or 9-oxadecyl.

If R⁰ denotes an alkyl radical in which one CH₂ group has been replacedby —CH═CH—, this may be straight-chain or branched. It is preferablystraight-chain and has 2 to 10 C atoms. Accordingly, it denotes, inparticular, vinyl, prop-1- or -2-enyl, but-1-, -2- or -3-enyl, pent-1-,-2-, -3- or -4-enyl, hex-1-, -2-, -3-, -4- or -5-enyl, hept-1-, -2-,-3-, -4-, -5- or -6-enyl, oct-1-, -2-, -3-, -4-, -5-, -6- or -7-enyl,non-1-, -2-, -3-, -4-, -5-, -6-, -7- or -8-enyl, dec-1-, -2-, -3-, -4-,-5-, -6-, -7-, -8- or -9-enyl. These radicals may also be mono- orpolyhalogenated.

If R⁰ denotes an alkyl or alkenyl radical which is at leastmonosubstituted by halogen, this radical is preferably straight-chain,and halogen is preferably F or Cl. In the case of polysubstitution,halogen is preferably F. The resultant radicals also includeperfluorinated radicals. In the case of monosubstitution, the fluorineor chlorine substituent may be in any desired position, but ispreferably in the ω-position.

Particular preference is given to compounds of the formulae IV to VIIIin which X⁰ denotes F or OCF₃.

Further preferred embodiments are indicated below:

-   -   The medium additionally comprises one or more neutral compounds        of the formulae II and/or III

-   -   in which    -   A denotes 1,4-phenylene or trans-1,4-cyclohexylene,    -   a is 0 or 1, and    -   R³ denotes alkenyl having 2 to 9 C atoms,    -   and R⁴ has the meaning indicated for R⁰ in formula I and        preferably denotes alkyl having 1 to 12 C atoms or alkenyl        having 2 to 9 C atoms.    -   The compounds of the formula II are preferably selected from the        following formulae:

-   -   in which R^(3a) and R^(4a) each, independently of one another,        denote H, CH₃, C₂H₅ or C₃H₇, and “alkyl” denotes a        straight-chain alkyl group having 1 to 8 C atoms. Particular        preference is given to compounds of the formulae IIa and IIf, in        particular in which R^(3a) denotes H or CH₃, and compounds of        the formula IIc, in particular in which R^(3a) and R^(4a) denote        H, CH₃ or C₂H₅.    -   Preference is furthermore given to compounds of the formula II        which have a non-terminal double bond in the alkenyl side chain:

-   -   Very particularly preferred compounds of the formula II are the        compounds of the formulae

-   -   The compounds of the formula III are preferably selected from        the following formulae:

-   -   in which “alkyl” and R^(3a) have the meanings indicated above,        and R^(3a) preferably denotes H or CH₃. Particular preference is        given to compounds of the formula IIIb;    -   The medium preferably additionally comprises one or more        compounds selected from the following formulae:

-   -   in which    -   R⁰ has the meanings indicated in formula I, and    -   X⁰ denotes F, Cl, a mono- or polyfluorinated alkyl or alkoxy        radical having 1 to 6 C atoms, a mono- or polyfluorinated        alkenyl or alkenyloxy radical having 2 to 6 C atoms,    -   Y¹⁻⁶ each, independently of one another, denote H or F,    -   Z⁰ denotes —C₂H₄—, —(CH₂)₄—, —CH═CH—, —CF═CF—, —C₂F₄—, —CH₂CF₂—,        —CF₂CH₂—, —CH₂O—, —OCH₂—, —COO—, —CF₂O— or —OCF₂—, in the        formulae V and VI also a single bond, and    -   r denotes 0 or 1.    -   In the above formulae, X⁰ is preferably F, Cl or a mono- or        polyfluorinated alkyl or alkoxy radical having 1, 2 or 3 C atoms        or a mono- or polyfluorinated alkenyl or alkenyloxy radical        having 2 or 3 C atoms. X⁰ is particularly preferably F, Cl, CF₃,        CHF₂, OCF₃, OCHF₂, OCFHCF₃, OCFHCHF₂, OCFHCHF₂, OCF₂CH₃,        OCF₂CHF₂, OCF₂CHF₂, OCF₂CF₂CHF₂, OCF₂CF₂CH₂F, OCFHCF₂CF₃,        OCFHCF₂CHF₂, OCH═CF₂, OCF═CF₂, OCF₂CHFCF₃, OCF₂CF₂CF₃,        OCF₂CF₂CCIF₂, OCCIFCF₂CF₃, CF═CF₂, CF═CHF, OCH═CF₂, OCF═CF₂ or        CH═CF₂. X⁰ very particularly preferably denotes F or OCF₃.    -   In the compounds of the formulae IV to VIII, X⁰ preferably        denotes F or OCF₃, furthermore OCHF₂, CF₃, CF₂H, Cl, OCH═CF₂. R⁰        is preferably straight-chain alkyl or alkenyl having up to 6 C        atoms.    -   The compounds of the formula IV are preferably selected from the        following formulae:

-   -   in which R⁰ and X⁰ have the meanings indicated above.    -   In formula IV, R⁰ preferably denotes alkyl having 1 to 8 C        atoms, and X⁰ preferably denotes F, Cl, OCHF₂ or OCF₃,        furthermore OCH═CF₂. In the compound of the formula IVb, R⁰        preferably denotes alkyl or alkenyl. In the compound of the        formula IVd, X⁰ preferably denotes Cl, furthermore F.    -   The compounds of the formula V are preferably selected from the        formulae Va to Vj:

-   -   in which R⁰ and X⁰ have the meanings indicated above. In formula        V, R⁰ preferably denotes alkyl having 1 to 8 C atoms, and X⁰        preferably denotes F;    -   The medium comprises one or more compounds of the formula VI-1

-   -   particularly preferably those selected from the following        formulae:

-   -   in which R⁰ and X⁰ have the meanings indicated above. In formula        VI, R⁰ preferably denotes alkyl having 1 to 8 C atoms, and X⁰        preferably denotes F, furthermore OCF₃.    -   The medium comprises one or more compounds of the formula VI-2

-   -   particularly preferably those selected from the following        formulae:

-   -   in which R⁰ and X⁰ have the meanings indicated above.    -   In formula VI, R⁰ preferably denotes alkyl having 1 to 8 C        atoms, and X⁰ preferably denotes F;    -   The medium preferably comprises one or more compounds of the        formula VII in which Z⁰ denotes —CF₂O—, —CH₂CH₂— or —COO—,        particularly preferably those selected from the following        formulae:

-   -   in which R⁰ and X⁰ have the meanings indicated above. In formula        VII, R⁰ preferably denotes alkyl having 1 to 8 C atoms, and X⁰        preferably denotes F, furthermore OCF₃.    -   The compounds of the formula VIII are preferably selected from        the following formulae:

-   -   in which R⁰ and X⁰ have the meanings indicated above. In formula        VIII, R⁰ preferably denotes a straight-chain alkyl radical        having 1 to 8 C atoms. X⁰ preferably denotes F.    -   The medium additionally comprises one or more compounds of the        following formula:

-   -   in which R⁰, X⁰, Y¹ and Y² have the meaning indicated above, and

each, independently of one another, denote

-   -   where rings A and B do not both simultaneously denote        cyclohexylene;    -   The compounds of the formula IX are preferably selected from the        following formulae:

-   -   in which R⁰ and X⁰ have the meanings indicated above. In formula        IX, R⁰ preferably denotes alkyl having 1 to 8 C atoms, and X⁰        preferably denotes F. Particular preference is given to        compounds of the formula IXa;    -   The medium additionally comprises one or more compounds selected        from the following formulae:

-   -   in which R⁰, X⁰ and Y¹⁻⁴ have the meaning indicated in formula        I, and

each, independently of one another, denote

-   -   The compounds of the formulae X and XI are preferably selected        from the following formulae:

-   -   in which R⁰ and X⁰ have the meanings indicated above. R⁰        preferably denotes alkyl having 1 to 8 C atoms, and X⁰        preferably denotes F. Particularly preferred compounds are those        in which Y¹ denotes F and Y² denotes H or F, preferably F;    -   The medium additionally comprises one or more compounds of the        following formula:

-   -   in which R¹ and R² each, independently of one another, denote        n-alkyl, alkoxy, oxaalkyl, fluoroalkyl, alkenyloxy or alkenyl,        each having up to 9 C atoms, and preferably each, independently        of one another, denote alkyl having 1 to 8 C atoms. Y¹ denotes H        or F.    -   Preferred compounds of the formula XII are the compounds of the        formulae

-   -   in which    -   alkyl and alkyl* each, independently of one another, denote a        straight-chain alkyl radical having 1 to 6 C atoms, and    -   alkenyl and    -   alkenyl* each, independently of one another, denote a        straight-chain alkenyl radical having 2 to 6 C atoms.    -   The medium additionally comprises one or more compounds selected        from the following formulae:

-   -   in which R⁰, X⁰, Y¹ and Y² have the meanings indicated above. R⁰        preferably denotes alkyl having 1 to 8 C atoms, and X⁰        preferably denotes F or Cl;    -   The compounds of the formulae XIII and XIV are preferably        selected from the compounds of the formulae

-   -   in which R⁰ and X⁰ have the meanings indicated above. R⁰        preferably denotes alkyl having 1 to 8 C atoms. In the compounds        of the formula XIII, X⁰ preferably denotes F or Cl.    -   The medium additionally comprises one or more compounds of the        formulae D1, D2 and/or D3:

-   -   in which Y¹, Y², R⁰ and X⁰ have the meaning indicated above. R⁰        preferably denotes alkyl having 1 to 8 C atoms, and X⁰        preferably denotes F.    -   Particular preference is given to compounds of the formulae

-   -   in which R⁰ has the meanings indicated above and preferably        denotes straight-chain alkyl having 1 to 6 C atoms, in        particular C₂H₅, n-C₃H₇ or n-O₅H₁₁.    -   The medium additionally comprises one or more compounds of the        following formula:

-   -   in which Y¹, R¹ and R² have the meaning indicated above. R¹ and        R² preferably each, independently of one another, denote alkyl        having 1 to 8 C atoms;    -   The medium additionally comprises one or more compounds of the        following formula:

-   -   in which X⁰, Y¹ and Y² have the meanings indicated above, and        “alkenyl” denotes C₂₋₇-alkenyl. Particular preference is given        to compounds of the following formula:

-   -   in which R^(3a) has the meaning indicated above and preferably        denotes H;    -   The medium additionally comprises one or more tetracyclic        compounds selected from the formulae XIX to XXVII:

-   -   in which Y¹⁻⁴, R⁰ and X⁰ each, independently of one another,        have one of the meanings indicated above. X⁰ is preferably F,        Cl, CF₃, OCF₃ or OCHF₂. R⁰ preferably denotes alkyl, alkoxy,        oxaalkyl, fluoroalkyl or alkenyl, each having up to 8 C atoms.    -   In the formulae mentioned above and below,

preferably denotes

-   -   R⁰ is preferably straight-chain alkyl or alkenyl having 2 to 7 C        atoms;    -   X⁰ is preferably F, furthermore OCF₃, C₁ or CF₃;    -   The medium preferably comprises one, two or three compounds of        the formula I;    -   The medium preferably comprises one or more compounds selected        from the group of the compounds of the formulae I, II, III, V,        VI-1, VI-2, XII, XIII, XIV, XVII, XXIII, XXV;    -   The medium preferably comprises one or more compounds of the        formula VI-1;    -   The medium preferably comprises one or more compounds of the        formula VI-2;    -   The medium preferably comprises 1-25% by weight, preferably        2-20% by weight, particularly preferably 2-15% by weight, of        compounds of the formula I;    -   The proportion of compounds of the formulae II-XXVII in the        mixture as a whole is preferably 20 to 99% by weight;    -   The medium preferably comprises 25-80% by weight, particularly        preferably 30-70% by weight, of compounds of the formulae II        and/or III;    -   The medium preferably comprises 5-40% by weight, particularly        preferably 10-30% by weight, of compounds of the formula V;    -   The medium preferably comprises 3-30% by weight, particularly        preferably 6-25% by weight, of compounds of the formula VI-1;    -   The medium preferably comprises 2-30% by weight, particularly        preferably 4-25% by weight, of compounds of the formula VI-2;    -   The medium comprises 5-40% by weight, particularly preferably        10-30% by weight, of compounds of the formula XII;    -   The medium preferably comprises 1-25% by weight, particularly        preferably 2-15% by weight, of compounds of the formula XIII;    -   The medium preferably comprises 5-45% by weight, particularly        preferably 10-35% by weight, of compounds of the formula XIV;    -   The medium preferably comprises 1-20% by weight, particularly        preferably 2-15% by weight, of compounds of the formula XVI;    -   The medium additionally comprises one or more compounds of the        formulae St-1 to St-3:

-   -   in which R⁰, Y⁰, Y² and X⁰ have the meanings indicated above. R⁰        preferably denotes straight-chain alkyl, preferably having 1-6 C        atoms. X⁰ is preferably F or OCF₃. Y¹ preferably denotes F. Y²        preferably denotes F. Preference is furthermore given to        compounds in which Y¹═F and Y²═H. The compounds of the formulae        St-1 to St-3 are preferably employed in the mixtures according        to the invention in concentrations of 3-30% by weight, in        particular 5-25% by weight.    -   The medium additionally comprises one or more pyrimidine or        pyridine compounds of the formulae Py-1 to Py-5:

-   -   in which R⁰ is preferably straight-chain alkyl having 2-5 C        atoms. x denotes 0 or 1, preferably x=1. Preferred mixtures        comprise 3-30% by weight, in particular 5-20% by weight, of this        (these) pyri(mi)dine compound(s).

It has been found that even a relatively small proportion of compoundsof the formula I mixed with conventional liquid-crystal materials, butin particular with one or more compounds of the formulae II to XXVII,results in a significant increase in the light stability and in lowbirefringence values, with broad nematic phases with low smectic-nematictransition temperatures being observed at the same time, improving theshelf life. At the same time, the mixtures exhibit very low thresholdvoltages and very good values for the VHR on exposure to UV.

The term “alkyl” or “alkyl*” in this application encompassesstraight-chain and branched alkyl groups having 1-7 carbon atoms, inparticular the straight-chain groups methyl, ethyl, propyl, butyl,pentyl, hexyl and heptyl. Groups having 1-6 carbon atoms are generallypreferred.

The term “alkenyl” or “alkenyl*” in this application encompassesstraightchain and branched alkenyl groups having 2-7 carbon atoms, inparticular the straight-chain groups. Preferred alkenyl groups areC₂-C₇-1E-alkenyl, C₄-C₇-3E-alkenyl, C₅-C₇-4-alkenyl, C₆-C₇-5-alkenyl andC₇-6-alkenyl, in particular C₂-C₇-1E-alkenyl, C₄-C₇-3E-alkenyl andC₅-C₇-4-alkenyl. Examples of particularly preferred alkenyl groups arevinyl, 1E-propenyl, 1E-butenyl, 1E-pentenyl, 1E-hexenyl, 1E-heptenyl,3-butenyl, 3E-pentenyl, 3E-hexenyl, 3E-heptenyl, 4-pentenyl, 4Z-hexenyl,4E-hexenyl, 4Z-heptenyl, 5-hexenyl, 6-heptenyl and the like. Groupshaving up to 5 carbon atoms are generally preferred.

The term “fluoroalkyl” in this application encompasses straight-chaingroups having at least one fluorine atom, preferably a terminalfluorine, i.e. fluoromethyl, 2-fluoroethyl, 3-fluoropropyl,4-fluorobutyl, 5-fluoropentyl, 6-fluorohexyl and 7-fluoroheptyl.However, other positions of the fluorine are not excluded.

The term “oxaalkyl” or “alkoxy” in this application encompassesstraight-chain radicals of the formula C_(n)H_(2n+1)—O—(CH₂)_(m), inwhich n and m each, independently of one another, denote 1 to 6. m mayalso denote 0. Preferably, n=1 and m=1-6 or m=0 and n=1-3.

Through a suitable choice of the meanings of R⁰ and X⁰, the addressingtimes, the threshold voltage, the steepness of the transmissioncharacteristic lines, etc., can be modified in the desired manner. Forexample, 1E-alkenyl radicals, 3E-alkenyl radicals, 2E-alkenyloxyradicals and the like generally result in shorter addressing times,improved nematic tendencies and a higher ratio between the elasticconstants k₃₃ (bend) and k₁₁ (splay) compared with alkyl and alkoxyradicals. 4-Alkenyl radicals, 3-alkenyl radicals and the like generallygive lower threshold voltages and lower values of k₃₃/k₁₁ compared withalkyl and alkoxy radicals. The mixtures according to the invention aredistinguished, in particular, by high K₁ values and thus havesignificantly faster response times than the mixtures from the priorart.

The optimum mixing ratio of the compounds of the above-mentionedformulae depends substantially on the desired properties, on the choiceof the components of the above-mentioned formulae and on the choice ofany further components that may be present.

Suitable mixing ratios within the range indicated above can easily bedetermined from case to case.

The total amount of compounds of the above-mentioned formulae in themixtures according to the invention is not crucial. The mixtures cantherefore comprise one or more further components for the purposes ofoptimisation of various properties. However, the observed effect on thedesired improvement in the properties of the mixture is generallygreater, the higher the total concentration of compounds of theabove-mentioned formulae.

In a particularly preferred embodiment, the media according to theinvention comprise compounds of the formulae IV to VIII in which X⁰denotes F, OCF₃, OCHF₂, OCH═CF₂, OCF═CF₂ or OCF₂—CF₂H. A favourablesynergistic action with the compounds of the formula I results inparticularly advantageous properties. In particular, mixtures comprisingcompounds of the formulae I and VI, or I and XI, or I and VI and XI aredistinguished by their low threshold voltages.

The individual compounds of the above-mentioned formulae and thesubformulae thereof which can be used in the media according to theinvention are either known or can be prepared analogously to the knowncompounds.

The invention also relates to electro-optical displays, such as, forexample, TN, STN, TFT, OCB, IPS, FFS or MLC displays, having twoplane-parallel outer plates, which, together with a frame, form a cell,integrated non-linear elements for switching individual pixels on theouter plates, and a nematic liquid-crystal mixture having positivedielectric anisotropy and high specific resistance located in the cell,which contain media of this type, and to the use of these media forelectro-optical purposes.

The liquid-crystal mixtures according to the invention enable asignificant broadening of the available parameter latitude. Theachievable combinations of clearing point, viscosity at low temperature,thermal and UV stability and high optical anisotropy are far superior toprevious materials from the prior art.

The mixtures according to the invention are particularly suitable formobile applications and high-Δn TFT applications, such as, for example,PDAs, notebooks, LCD TVs and monitors.

The liquid-crystal mixtures according to the invention, while retainingthe nematic phase down to −20° C. and preferably down to −30° C.,particularly preferably down to −40° C., and the clearing point ≧70° C.,preferably ≧75° C., at the same time allow rotational viscosities γ_(i)of ≦120 mPa·s, particularly preferably ≦100 mPa·s, to be achieved,enabling excellent MLC displays having fast response times to beachieved.

The dielectric anisotropy Δ∈ of the liquid-crystal mixtures according tothe invention is preferably ≧+5, particularly preferably ≧+10. Inaddition, the mixtures are characterised by low operating voltages. Thethreshold voltage of the liquid-crystal mixtures according to theinvention is preferably ≦1.5 V, in particulars ≦1.2 V.

The birefringence Δn of the liquid-crystal mixtures according to theinvention is preferably ≧0.08, in particular ≧0.10 and very particularlypreferably ≧0.11.

The nematic phase range of the liquid-crystal mixtures according to theinvention preferably has a width of at least 90°, in particular at least100°. This range preferably extends at least from −25° C. to +69° C.

If the mixtures according to the invention are used in FFS applications,the mixtures preferably have a value for the dielectric anisotropy of3-12 and a value for the optical anisotropy of 0.07-0.13.

It goes without saying that, through a suitable choice of the componentsof the mixtures according to the invention, it is also possible forhigher clearing points (for example above 100° C.) to be achieved athigher threshold voltages or lower clearing points to be achieved atlower threshold voltages with retention of the other advantageousproperties. At viscosities correspondingly increased only slightly, itis likewise possible to obtain mixtures having a higher Δ∈ and thus lowthresholds. The MLC displays according to the invention preferablyoperate at the first Gooch and Tarry transmission minimum [C. H. Goochand H. A. Tarry, Electron. Lett. 10, 2-4, 1974; C. H. Gooch and H. A.Tarry, Appl. Phys., Vol. 8, 1575-1584, 1975], where, besidesparticularly favourable electro-optical properties, such as, forexample, high steepness of the characteristic line and low angledependence of the contrast (German patent 30 22 818), lower dielectricanisotropy is sufficient at the same threshold voltage as in ananalogous display at the second minimum. This enables significantlyhigher specific resistance values to be achieved using the mixturesaccording to the invention at the first minimum than in the case ofmixtures comprising cyano compounds. Through a suitable choice of theindividual components and their proportions by weight, the personskilled in the art is able to set the birefringence necessary for apre-specified layer thickness of the MLC display using simple routinemethods.

Measurements of the voltage holding ratio (HR) [S. Matsumoto et al.,Liquid Crystals 5, 1320 (1989); K. Niwa et al., Proc. SID Conference,San Francisco, June 1984, p. 304 (1984); G. Weber et al., LiquidCrystals 5, 1381 (1989)] have shown that mixtures according to theinvention comprising compounds of the formula I exhibit a significantlysmaller decrease in the HR on UV exposure than analogous mixturescomprising cyanophenylcyclohexanes of the formula

or esters of the formula

instead of the compounds of the formula I.

The light stability and UV stability of the mixtures according to theinvention are considerably better, i.e. they exhibit a significantlysmaller decrease in the HR on exposure to light or UV. Even lowconcentrations of the compounds (<10% by weight) of the formula I in themixtures increase the HR by 6% or more compared with mixtures from theprior art.

The construction of the MLC display according to the invention frompolarisers, electrode base plates and surface-treated electrodescorresponds to the usual design for displays of this type. The termusual design is broadly drawn here and also encompasses all derivativesand modifications of the MLC display, in particular including matrixdisplay elements based on poly-Si TFTs or MIM.

A significant difference between the displays according to the inventionand the hitherto conventional displays based on the twisted nematic cellconsists, however, in the choice of the liquid-crystal parameters of theliquid-crystal layer.

The liquid-crystal mixtures which can be used in accordance with theinvention are prepared in a manner conventional per se, for example bymixing one or more compounds of the formula I with one or more compoundsof the formulae II-XXVII or with further liquid-crystalline compoundsand/or additives. In general, the desired amount of the components usedin the smaller amount is dissolved in the components making up theprincipal constituent, advantageously at elevated temperature. It isalso possible to mix solutions of the components in an organic solvent,for example in acetone, chloroform or methanol, and to remove thesolvent again, for example by distillation, after thorough mixing.

The dielectrics may also comprise further additives known to the personskilled in the art and described in the literature, such as, forexample, UV stabilisers, such as Tinuvin® from Ciba Chemicals,antioxidants, free-radical scavengers, nanoparticles, etc. For example,0-15% of pleochroic dyes or chiral dopants can be added. Suitablestabilisers and dopants are mentioned below in Tables C and D.

In the present application and in the examples below, the structures ofthe liquid-crystal compounds are indicated by means of acronyms, thetransformation into chemical formulae taking place in accordance withTable A. All radicals C_(n)H_(2n+1) and C_(m)H_(2m+1) are straight-chainalkyl radicals having n and m C atoms respectively; n, m and k areintegers and preferably denote 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 or12. The coding in Table B is self-evident. In Table A, only the acronymfor the parent structure is indicated. In individual cases, the acronymfor the parent structure is followed, separated by a dash, by a code forthe substituents R¹*, R²*, L¹* and L²*:

Code for R¹*, R²*, L¹*, L²*, L³* R¹* R²* L¹* L²* nm C_(n)H_(2n+1)C_(m)H_(2m+1) H H nOm C_(n)H_(2n+1) OC_(m)H_(2m+1) H H nO.mOC_(n)H_(2n+1) C_(m)H_(2m+1) H H n C_(n)H_(2n+1) CN H H nN.FC_(n)H_(2n+1) CN F H nN.F.F C_(n)H_(2n+1) CN F F nF C_(n)H_(2n+1) F H HnCl C_(n)H_(2n+1) Cl H H nOF OC_(n)H_(2n+1) F H H nF.F C_(n)H_(2n+1) F FH nF.F.F C_(n)H_(2n+1) F F F nOCF₃ C_(n)H_(2n+1) OCF₃ H H nOCF₃.FC_(n)H_(2n+1) OCF₃ F H n-Vm C_(n)H_(2n+1) —CH═CH—C_(m)H_(2m+1) H H nV-VmC_(n)H_(2n+1)—CH═CH— —CH═CH—C_(m)H_(2m+1) H H

Preferred mixture components are shown in Tables A and B.

TABLE A

PYP

PYRP

BCH

CBC

CCH

CCP

CPTP

CEPTP

ECCP

CECP

EPCH

PCH

CH

PTP

CCPC

CP

BECH

EBCH

CPC

B

FET-nF

CGG

CGU

CFU

TABLE B

Particular preference is given to liquid-crystalline mixtures which,besides the compounds of the formula I, comprise at least one, two,three, four or more compounds from Table B.

TABLE C Table C indicates possible dopants which are generally added tothe mixtures according to the invention. The mixtures preferablycomprise 0-10% by weight, in particular 0.01-5% by weight andparticularly preferably 0.01-3% by weight of dopants.

TABLE D Stabilisers which can be added, for example, to the mixturesaccording to the invention in amounts of 0-10% by weight are mentionedbelow.

EXAMPLES

“Conventional work-up” means: water is added if necessary, the mixtureis extracted with methylene chloride, diethyl ether, methyl tert-butylether or toluene, the phases are separated, the organic phase is driedand evaporated, and the product is purified by distillation underreduced pressure or crystallisation and/or chromatography. The followingabbreviations are used:

DAST diethylaminosulfur trifluorideDCC dicyclohexylcarbodiimideDDQ dichlorodicyanobenzoquinoneDIBALH diisobutylaluminium hydrideKOT potassium tertiary-butoxideRT room temperatureTHF tetrahydrofuranpTSOH p-toluenesulfonic acidTMEDA tetramethylethylenediamine

Example 1

Step 1.1

84.996 mmol of sodium hydride suspension (60% suspension in paraffinoil) are initially introduced in 210 ml of THF (dried) and cooled toabout 0° C., and the solution of 1 in 240 ml of THF (dried) is addeddropwise at this temperature. The previously white suspension becomes apale-yellow clear solution. The mixture is then stirred at roomtemperature for a further 1 hour. The reaction solution is subsequentlyheated at reflux (68° C.), and a water separator is installed, with theaid of which the THF is distilled off. After about 0.5 h, 3523.399 mmolof 1,3-dimethyl-2-imidazolidinone are added, and the reaction solutionis warmed to a bath temperature of about 138° C. After a post-stirringtime of 30 min. at 130° C., 84.996 mmol of 2,2,2-trifluoroethylp-toluenesulfonate are added. The mixture is subsequently stirredovernight at about 100° C.

Water and methyl tert-butyl ether are added to the reaction solution,which is then acidified using 25% HCl, and the phases are separated. Theaqueous phase is then extracted with methyl tert-butyl ether, and thecombined organic phases are washed with water and with sat. NaClsolution, dried over sodium sulfate, filtered with suction andevaporated. The residue is subsequently subjected to conventionalpurification, giving a white crystalline powder.

Step 1.2

68.692 mmol of 2 in 200 ml of THF (dried) are initially introduced inthe inertised apparatus and cooled to −70° C. with stirring. Lithiumdiisopropylamine, prepared from 206.1 mmol of diisopropylamine in 50 mlof THF and 208.848 mmol of butyllithium (15% solution in n-hexane), isthen added dropwise at max. −70° C., and the mixture is subsequentlystirred at −70° C. for a further 2 hours. The reaction mixture is thenwarmed to −30° C., and sat. sodium bicarbonate solution is added. Methyltert-butyl ether and water are added to the reaction mixture, which isthen washed until neutral. The combined organic phases are dried oversodium sulfate and evaporated on a Rotavapor. Finally, the mixture issubjected to conventional work-up, giving the product in the form ofwhite crystals. C 58 S_(A) 97 N 98.3 I;

Δn=0.1547; Δ∈=13.2; γ_(i)=129.

The following mixture examples are intended to explain the inventionwithout limiting it.

Above and below, percentage data denote percent by weight. Alltemperatures are indicated in degrees Celsius. m.p. denotes meltingpoint, cl.p.=clearing point. Furthermore, C=crystalline state, N=nematicphase, S=smectic phase and I=isotropic phase. The data between thesesymbols represent the transition temperatures. Furthermore,

-   -   Δn denotes the optical anisotropy at 589 nm and 20° C.,    -   γ₁ denotes the rotational viscosity (mPa·s) at 20° C.,    -   V₁₀ denotes the voltage (V) for 10% transmission (viewing angle        perpendicular to the plate surface), (threshold voltage),    -   Δ∈ denotes the dielectric anisotropy at 20° C. and 1 kHz        (Δ∈=∈_(∥)−∈_(⊥), where ∈_(∥) denotes the dielectric constant        parallel to the longitudinal axes of the molecules and ∈_(⊥)        denotes the dielectric constant perpendicular thereto).

The electro-optical data are measured in a TN cell at the 1st minimum(i.e. at a d·Δn value of 0.5 μm) at 20° C., unless expressly indicatedotherwise. The optical data are measured at 20° C., unless expresslyindicated otherwise. All physical properties are determined inaccordance with “Merck Liquid Crystals, Physical Properties of LiquidCrystals”, status November 1997, Merck KGaA, Germany, and apply for atemperature of 20° C., unless explicitly indicated otherwise.

Example 1

CC-3-V 29.00% Clearing point [° C.]: 75.5 APU-3-OXF 7.00% S→ N [° C.]:−30 ACQU-3-F 7.00% Δn [589 nm, 20° C.]: 0.1158 PUQU-3-F 15.00% Δε [kHz,20° C.]: +18.5 CCP-V-1 9.00% γ₁ [mPa · s, 20° C.]: 104 APUQU-2-F 7.00%V₁₀ [V]: 1.00 APUQU-3-F 7.00% PGUQU-3-F 7.00% CPGU-3-OT 4.00% CGUQU-3-F8.00%

Example 2

CC-3-V 26.00% Clearing point [° C.]: 74.5 PGP-2-3 2.00% S→ N [° C.]: −40ACQU-3-F 11.00% Δn [589 nm, 20° C.]: 0.1118 CCQU-3-F 10.00% Δε [kHz, 20°C.]: +18.0 APU-3-OXF 8.00% γ₁ [mPa · s, 20° C.]: 100 PUQU-3-F 14.00% V₁₀[V]: 1.00 CCP-V-1 6.00% APUQU-2-F 6.00% APUQU-3-F 6.00% PGUQU-3-F 8.00%CPGU-3-OT 3.00%

Example 3

CC-3-V 40.00% Clearing point [° C.]: 72.5 PGP-2-3 5.00% S→ N [° C.]: −40APU-3-OXF 7.00% Δn [589 nm, 20° C.]: 0.1090 PUQU-3-F 13.00% Δε [kHz, 20°C.]: +12.3 CCP-V-1 10.00% γ₁ [mPa · s, 20° C.]: 70 CDUQU-3-F 7.00% V₁₀[V]: 1.19 APUQU-2-F 5.00% APUQU-3-F 6.00% PGUQU-3-F 7.00%

Example 4

CC-3-V 37.00% PGP-2-3 2.00% CCQU-3-F 7.00% APU-3-OXF 7.00% PUQU-3-F14.00% CCP-V-1 11.00% APUQU-2-F 6.00% APUQU-3-F 6.00% PGUQU-3-F 8.00%CPGU-3-OT 2.00%

Example 5

PGP-2-3 3.00% Clearing point [° C.]: 76.5 PGP-2-4 4.00% Δn [589 nm, 20°C.]: 0.1038 CCP-V-1 5.00% Δε [kHz, 20° C.]: +8.0 CC-3-V 47.00% γ₁ [mPa ·s, 20° C.]: 58 CC-3-V1 8.00% APUQU-2-F 8.00% APUQU-3-F 8.00% PGUQU-3-F8.00% APU-3-OXF 9.00%

Example 6

CC-3-V 40.00% Clearing point [° C.]: 74.5 APU-3-OXF 7.00% S→ N [° C.]:−40 PGU-3-F 4.00% Δn [589 nm, 20° C.]: 0.1112 PUQU-3-F 12.00% Δε [kHz,20° C.]: +12.1 CCP-V-1 13.00% γ₁ [mPa · s, 20° C.]: 72 APUQU-2-F 6.00%V₁₀ [V]: 1.21 APUQU-3-F 6.00% PGUQU-3-F 8.00% CPGU-3-OT 4.00%

Example 7

PGP-2-4 5.00% CCP-V-1 5.00% CC-3-V 48.00% CC-3-V1 9.00% APUQU-2-F 8.00%APUQU-3-F 9.00% PGUQU-3-F 9.00% APU-3-OXF 7.00%

Example 8

CC-3-V 49.00% CCP-V-1 11.00% PUQU-3-F 4.00% PGP-2-4 2.00% APUQU-2-F9.00% APUQU-3-F 9.00% PGUQU-3-F 9.00% APU-3-OXF 7.00%

Example 9

PUQU-3-F 3.00% Clearing point [° C.]: 83 PGUQU-3-F 6.00% Δn [589 nm, 20°C.]: 0.1026 PGP-2-4 4.00% Δε [kHz, 20° C.]: +7.9 CCP-V-1 14.00% γ₁ [mPa· s, 20° C.]: 63 CCP-V2-1 4.00% CC-3-V 46.00% APUQU-2-F 8.00% APUQU-3-F8.00% APU-3-OXF 7.00%

Example 10

GPQU-3-F 15.00% Clearing point [° C.]: 69 CC-3-V1 6.00% Δn [589 nm, 20°C.]: 0.1033 CC-3-V 45.50% Δε [kHz, 20° C.]: +2.9 CCP-V-1 15.00% γ₁ [mPa· s, 20° C.]: 45 PGP-2-3 5.50% PGP-2-4 5.00% PGP-2-5 5.00% APU-3-OXF3.00%

Examples 1-9 are preferably suitable for TN-TFT applications, while themixture from Example 10 is an IPS mixture.

1. Liquid-crystalline medium, characterised in that it comprises one ormore compounds of the formula I

in which R⁰ denotes an alkyl or alkoxy radical having 1 to 15 C atoms,where, in addition, one or more CH₂ groups in these radicals may each bereplaced, independently of one another, by —C≡C—, —CF₂O—, —CH═CH—,

—O—, —CO—O— or —O—CO— in such a way that O atoms are not linked directlyto one another, and in which, in addition, one or more H atoms may bereplaced by halogen,

denotes

L¹⁻⁴ each, independently of one another, denote H or F. 2.Liquid-crystalline medium according to claim 1, characterised in that itadditionally comprises one or more compounds of the formulae II and/orIII:

in which A denotes 1,4-phenylene or trans-1,4-cyclohexylene, a denotes 0or 1, R³ denotes alkenyl having 2 to 9 C atoms, and R⁴ has the meaningsindicated for R⁰ in claim
 1. 3. Liquid-crystalline medium according toclaim 1, characterised in that it additionally comprises one or morecompounds selected from the compounds of the formulae:

in which R^(3a) and R^(4a) each, independently of one another, denote H,CH₃, C₂H₅ or C₃H₇, and “alkyl” denotes a straight-chain alkyl grouphaving 1 to 8 C atoms.
 4. Liquid-crystalline medium according to claim1, characterised in that it additionally comprises one or more compoundsselected from the compounds of the formulae IV to VIII:

in which R⁰ denotes an alkyl or alkoxy radical having 1 to 15 C atoms,where, in addition, one or more CH₂ groups in these radicals may each bereplaced, independently of one another, by —C≡C—, —CF₂O—, —CH═CH—,

—O—, —CO—O— or —O—CO— in such a way that O atoms are not linked directlyto one another, and in which, in addition, one or more H atoms may bereplaced by halogen, X⁰ denotes F, Cl, a mono- or polyfluorinated alkylor alkoxy radical having 1 to 6 C atoms, a mono- or polyfluorinatedalkenyl or alkenyloxy radical having 2 to 6 C atoms, Y¹⁻⁶ each,independently of one another, denote H or F, Z⁰ denotes —C₂H₄—,—(CH₂)₄—, —CH═CH—, —CF═CF—, —C₂F₄—, —CH₂CF₂—, —CF₂CH₂—, —CH₂O—, —OCH₂—,—COO—, —CF₂O— or —OCF₂—, in the formulae V and VI also a single bond,and r denotes 0 or
 1. 5. Liquid-crystalline medium according to claim 1, characterised in that it additionally comprises one or more compoundsselected from the compounds of the formulae Va to Vj:

in which R⁰ and X⁰ have the meanings indicated in claim
 1. 6.Liquid-crystalline medium according to claim 1, characterised in that itadditionally comprises one or more compounds selected from the compoundsof the formulae VI-1a to VI-1d:

in which R⁰ and X⁰ have the meanings indicated in claim
 1. 7.Liquid-crystalline medium according to claim 1, characterised in that itadditionally comprises one or more compounds selected from the compoundsof the formulae VI-2a to VI-2f:

in which R⁰ and X⁰ have the meanings indicated in claim
 1. 8.Liquid-crystalline medium according to claim 1, characterised in that itadditionally comprises one or more compounds selected from compounds ofthe formulae X and/or XI:

in which R⁰ and X⁰ have the meanings indicated in claim 1, Y¹⁻⁴ each,independently of one another, denote H or F, and

each, independently of one another, denote


9. Liquid-crystalline medium according to claim 1, characterised in thatit additionally comprises one or more compounds selected from compoundsof the formula XII

in which R¹ and R² each, independently of one another, denote n-alkyl,alkoxy, oxaalkyl, fluoroalkyl, alkenyloxy or alkenyl, each having up to9 C atoms, and Y¹ denotes H or F.
 10. Liquid-crystalline mediumaccording to claim 4, characterised in that it additionally comprisesone or more compounds selected from the following formulae:

in which R⁰, X⁰, Y¹ and Y² have the meanings indicated in claim
 4. 11.Liquid-crystalline medium according to claim 1, characterised in that itcomprises 1-25% by weight of compounds of the formula I. 12.Liquid-crystalline medium according to claim 1, characterised in that itadditionally comprises one or more UV stabilisers and/or antioxidants.13. Use of a liquid-crystalline medium according to claim 1 forelectro-optical purposes.
 14. Electro-optical liquid-crystal displaycontaining a liquid-crystalline medium according to claim
 1. 15. Processfor the preparation of a liquid-crystalline medium according to claim 1,characterised in that one or more compounds of the formula I are mixedwith at least one further mesogenic compound and optionally additives.